Factored out globals and refactored helpers

Refactored global helpers into a Vagrant::Environment class that provides
functionality related to a given mw-vagrant environment to plugins and the
Vagrantfile itself. The greater level of encapsulation should help to
avoid global-related race conditions like the one reported in bug 67445.

Bug: 67445
Change-Id: Ibde79a28147f5c2800a75446e0d297ddb7aedef0